Assimilate allocation by rice and carbon stabilisation in soil: effect of water management and phosphorus fertilisation

Abstract

Water and nutrient management influences the allocation and stabilisation of newly assimilated carbon (C) in paddy soils. This study aimed to determine the belowground allocation of C assimilated by rice and the subsequent C stabilisation in soil aggregates and as mineral-organic associates depending on combined alternate wetting and drying (AWD) versus continuous flooding (CF) and P fertilisation. We continuously labelled rice plants in 13CO2 atmosphere under AWD versus CF water management, and at two P fertilisation levels (0 or 80 mg P kg−1 soil). The 13C allocation to soil and its incorporation into the wet-sieved aggregate size classes and density fractions of the rhizosphere and bulk soils were analysed 6, 14, and 22 days after the labelling was started (D6, D14, and D22, respectively). Under both water regimes and P fertilisation levels, the proportion of photoassimilates was the highest in the silt- and clay-size aggregate classes and in the mineral-associated fraction. On D6 and D14, P fertilization resulted in smaller 13C incorporation into soil, independent of water management. In the rhizosphere soil, at D22, P fertilisation increased 13C incorporation over no P amendment in macroaggregates (>250 μm) by 32% (AWD) and 42% (CF), in microaggregates (250–53 μm) by 97% (CF), and in the silt + clay size class (<53 μm) by 83% (CF). Further, P fertilisation led to larger 13C incorporation into the rhizosphere soil light fraction (75% at AWD and 90% at CF) and dense fraction (38% and 45%, respectively), and into the bulk soil macroaggregates (71% and 78%, respectively). Phosphorus fertilisation increased the contents of recent photoassimilates in soil aggregate classes with longer residence time as well as of the particulate organic matter with the continuation of plant growth. This positive response of the stabilisation of recent plant photosynthates in soil to P fertilisation can increase the potential of paddy soil for C sequestration. This potential is not limited by the introduction of alternate wetting and drying water-saving technique.